Trevor Thomas went blind at age 36. He responded by hiking 18,000 miles.

I remember sitting at my kitchen counter, and my mom called and said, "Your brother's going blind." Watching him go through those eight months was the most agonizing time in our family's life. It changed everything. —Liz Thomas

On April 6, 2008, Trevor Thomas paced around a sign at the bottom of Georgia's Springer Mountain, scheming. Because the sign was engraved, he was able to tell that it read, "2.2 Black Gap Shelter." Precisely 2,178.5 miles north was Mt. Katahdin, Maine. This was the southern entrance to the Appalachian Trail. His sister, Liz Thomas, was there to pass him off to his hiking partner, who she believed would be showing up soon.

"Where the heck is this guy?" she asked.

"He'll be here any minute," Trevor lied. His partner had bailed days earlier, but he couldn't tell Liz that. She was his big sister. She'd never let him go alone. He racked his mind for a viable story. Then he had an idea. "Let me talk to the ranger," he said.

After a few minutes, Trevor returned from the register, which was located just up the trail. His lie ballooned. He told Liz that a ranger had found his partner's name and an accompanying note saying that he had begun hiking earlier in the day and that Trevor was to meet him at the Black Gap Shelter. She believed this. Now, Trevor and Liz just had to find a hiker to take him there.

For several hours they lingered in the parking lot, approaching every group starting out on the trail. "Excuse me," he would say, "my name's Trevor Thomas. I'm blind and I need to get to the Black Gap Shelter. Can I follow you there? I just want to walk behind someone." They asked 24 groups for help, and all of them refused. "You want me to be responsible for what?" someone said.

Frustrated, Trevor took a walk up the trail, which wound through a sparse forest. Back in the parking lot, Liz locked eyes with a hiker in his mid-20s named Kevin Rondeau and gave him the spiel: "Excuse me, my brother is blind, and he needs to get to the shanty two miles down the trail. Would you be willing to help him?" She pointed at Trevor, who could be seen pacing amid the trees. There was no hint of his blindness. He walked in swift, straight vectors. His trekking poles, which he used like antennae on the ground before each step, were a seamless part of his hiking uniform.

Rondeau said, "That guy is blind?"

"Yes," she answered.

Rondeau thought for a moment and said, "If that guy is actually blind, he can walk with me as long as he wants."

Thomas and Rondeau hiked together for a week before splitting up. For the rest of the trail, Thomas—now going by Zero/Zero, a trail alias playing on 20/20—floated between hiking partners, but he also hiked solo for long spans. On his first night alone, about two weeks into his journey, it snowed so much that the trail vanished. The next day, convinced that it would be a waste of fuel to melt snow, he spent four and a half hours crawling outward from his campsite in all directions looking for a buried stream to drink from.

Thomas had never backpacked before, yet he was relatively well prepared for the trail. He had spent a year going over the map, memorizing every checkpoint and the distances between with the help of a spirited young employee at a local gear shop. He learned to gauge his walking speed, which helped him calculate how far he'd traveled. He knew where to find water. He knew where to listen for openings in the trees—camping spots—and how to read engraved signs with his hands. When he came to a confusing junction, he assumed that whichever trail felt harder underfoot was the Appalachian, and he was usually right. Still, he would sometimes guess wrong and wander off track for hours. He fell constantly—he stopped counting at 3,000—and broke many bones, including several ribs that he had treated by a veterinarian in a Maine town with no doctor. To finish the trail, he walked 200 miles on a broken foot and with four cracked ribs.

It took him six months to reach Mt. Katahdin. As he came over its false summit, the fierce wind died, and he heard voices on the true summit a half mile away. Zigzagging through pitted snow, he followed the sounds. This was the end of the trail. The sun was out, but the air was freezing. As on any summit, sounds refused to reverberate—they disappeared into a void.

When he reached the top, he picked up a rock. As he handled it, he memorized the moment. The stone was like a photograph for him, the last of dozens he'd picked up on his journey north. This one was a deceptively light chunk of sediment—porous, covered in sharp bumps, and shaped like a railroad spike. He put it in his pocket. His Katahdin stone now sits in a box in his house with many other cherished rocks. He can pick up any one of them and describe the place where he found it.

Thomas borrowed a phone from a fellow summiter and called home. He spoke proudly to Liz and his mother, Judith, and was surprised by the warm congratulations of his father, Mark, a world-renowned flautist with whom he has a distant relationship. "That was the happiest I'd ever been," Thomas recalls. "I felt like I'd gotten my life back."

As Trevor went blind, he was basically going crazy, because he was seeing things that weren't there, and he wasn't seeing things that were there. One day he saw thousands of spiders on the floors and walls of his room. It was horrific. —Liz Thomas

He'd tell me, "Liz, get out of the way! There's people coming in the windows!" His eyes were basically disconnecting from his brain.
—Liz Thomas

When I entered Trevor Thomas's room at the Brevard, North Carolina, Holiday Inn last April, he was seated on the edge of a bed, his black Labrador retriever of seven months, Tennille, at his side. Wet camping gear hung in the bathroom, and the room smelled like a dirty sponge. Thomas and Tennille were two weeks into their attempted through-hike of the Mountains-to-Sea Trail, a little-known route that draws a 950-mile arc across the state, from the Smokies to the Outer Banks.

Since finishing the Appalachian Trail five years earlier, Thomas had tried to walk up the country twice more—on the Pacific Crest Trail, which he'd completed, and on the Continental Divide Trail, which, because of snow, he had not. He'd been filmed for several short documentaries and written about in newspapers and magazines. He had a foundation and Facebook fans and speaking engagements. He had people telling him to do a feature-length film and a book.

Now, he slumped forward, exhausted. It was 9:30 p.m. and he still had to organize, repack, check in with his shoe sponsor and his sock sponsor, dictate a Facebook update, and do an interview with a pair of undergraduate documentarians who had just set up a camera in his room. His left leg was numb. There was something wrong with his iPad, paid for by one of his sponsors, so he couldn't post photos to Facebook. Part of him wouldn't have minded leaving it behind, but he felt he owed something to anyone who believed in him. On top of all this, he had to start hiking early the next day—there were 700 miles to go on the Mountains-to-Sea Trail.

The two students adjusted the lighting from drab to bright. Tennille turned onto her back, went limp-wristed, swung her pendulous tail, and pressed her upside-down head into the leg of her owner. She seemed to sense that he just wanted to get this over with.

"Can we turn this off?" The male student pointed at the air-conditioning unit.

"You're going to have to tell me what 'this' is."

"Sorry. The air-conditioning."

"As long as you turn it back on, because I don't know how to do that." Thomas rubbed Tennille's belly with his whole hand. She stretched a leg across his lap.

Thomas had an athletic look—fat-free translucent skin over tubular veins, sinewy legs, prominent clavicles and sternum. His huge sunglasses made him look like a bug. "She's the only dog I know who sleeps on her back," he said, smiling down at her. When they'd first met, she had bonded with him immediately, and now he mocked her choice in a high, whiny tone: "'I don't want to go hiking tomorrow. I don't like getting all dirty. I wish I'd gotten someone infirm.'"

"Can we get you to talk about your logistics?" the male student asked. "Just give us an overview."

"Um, OK. Didn't we already talk about logistics?"

"What about all these bags?"

"You mean the food? Now that's the resupply. I don't know what there is to explain. It's in rows."

In a picture in his head, he could see this resupply. On the opposite bed were Tennille's 21 bags of kibble in one row, with 7 bags of M&Ms and 2 bags of cashews in another row. On the round table were dried mangoes, beef jerky, a mess kit, a camera, the iPad, batteries, smoking tobacco, and more M&Ms. All around the room, stuff sacks lined the wall. Nothing touched anything else.

The male student looked to his partner, a female student on the floor with a microphone, for guidance. She shrugged her shoulders. Tennille began to snore. "I think we have what we need," the male student finally said, and the girl nodded. After turning the air conditioner back on, they left.

"You seem frustrated," I said.

"It's past Tennille's bedtime. And I've got to deal with this stupid iPad thing that weighs five freaking pounds and I can't figure out how to work."

He tried the iPad a few more times but never figured it out, so he gave up and sent it home.